Widespread acquisition of antimicrobial resistance among Campylobacter isolates from UK retail poultry and evidence for clonal expansion of resistant lineages.

Department of Zoology, University of Oxford, South Parks Road, Oxford OX1 3PS, UK.

Abstract

BACKGROUND:

Antimicrobial resistance is increasing among clinical Campylobacter cases and is common among isolates from other sources, specifically retail poultry - a major source of human infection. In this study the antimicrobial susceptibility of isolates from a UK-wide survey of Campylobacter in retail poultry in 2001 and 2004-5 was investigated. The occurrence of phenotypes resistant to tetracycline, quinolones (ciprofloxacin and naladixic acid), erythromycin, chloramphenicol and aminoglycosides was quantified. This was compared with a phylogeny for these isolates based upon Multi Locus Sequence Typing (MLST) to investigate the pattern of antimicrobial resistance acquisition.

RESULTS:

Antimicrobial resistance was present in all lineage clusters, but statistical testing showed a non-random distribution. Erythromycin resistance was associated with Campylobacter coli. For all antimicrobials tested, resistant isolates were distributed among relatively distant lineages indicative of widespread acquisition. There was also evidence of clustering of resistance phenotypes within lineages; indicative of local expansion of resistant strains.

CONCLUSIONS:

These results are consistent with the widespread acquisition of antimicrobial resistance among chicken associated Campylobacter isolates, either through mutation or horizontal gene transfer, and the expansion of these lineages as a proportion of the population. As Campylobacter are not known to multiply outside of the host and long-term carriage in humans is extremely infrequent in industrialized countries, the most likely location for the proliferation of resistant lineages is in farmed chickens.

Permutation test results for the association of lineage with resistance phenotype for the tested antimicrobials. Comparison of a measure of association of resistant lineages with that expected by chance for (A) tetracycline, (B) naladixic acid, (C) ciprofloxacin, (D) erythromycin, (E) chloramphenicol. The arrows show the results from the data compared with frequency histograms of the scores from 10,000 permutations of the data which show the expected distribution of scores if no association exists. No comparison was made for aminoglycosides because too few isolates displayed resistance and so the test had no power.